1. Field of the Invention
The present invention relates generally to single-crystal growth techniques and, more particularly, to a coil for use in single-crystal growth utilizing a floating zone (FZ) method.
2. Description of the Related Art
A floating zone (FZ) method is one single-crystal growth method. In the floating zone method, a vertically held polycrystalline rod is partly transformed to melt at the bottom end by high-frequency induction heating and the bottom end is relatively moved to and fused with the tip of a seed crystal thin rod with a definite crystallographic orientation which is held under the polycrystalline rod coaxially thereto by means of high frequency induction heating and the tip coalesces with the melt and is partly molten. Then the melt is gradually moved toward the polycrystalline rod with a coil for the high-frequency induction heating, whereby a single crystal is successively grown.
In the single-crystal growth utilizing the floating zone method, for example, the coil shown in FIG. 4 is usually used.
A coil 1 is formed as an annular single-turn coil having a wedge-like configuration in vertical section. The coil 1 is also formed to have a thickness which progressively increases from inner circumference to outer circumference, so that the top side (or surface positioned on the polycrystalline side) is inclined downward in the radially inward direction.
When a single crystal is to be grown by using the coil 1, a high-frequency current is made to flow in the coil 1 and a coolant is supplied to the interior of the coil 1 to prevent overheating. Thus, the energy required for melting is given to the polycrystalline rod 2a while the overheating of the coil 1 is being prevented, thereby melting the polycrystalline rod 2a to form a molten zone. A single-crystal rod 2b is produced by progressively moving the molten zone toward the polycrystalline side together with the coil 1. The coil 1 is made of copper, silver or a combination of copper and silver plating on it, and its surface is finished as a rough surface.
However, the above-described conventional single-crystal growth technique utilizing the aforesaid coil 1 has a number of problems.
For example, the coil 1 has an inner circumferential end which is extremely reduced in thickness, and the area in which this inner circumferential end face opposes a semiconductor rod 2 is extremely limited. As a result, although the supply of energy to the neck portion of the molten zone of the semiconductor rod 2 can be restricted to a miniature area, the absolute amount of energy which can be supplied is also restricted and it becomes difficult to perfectly melt the semiconductor rod 2 up to the core thereof. Accordingly, a solid-phase polycrystalline region remains at or near the core of the semiconductor rod 2, and the crystallinity of the resulting single crystal may not be perfect. In this case, even if the polycrystalline rod typically has a molten-liquid outer portion, the interior remains unmolten. This remaining unmolten portion, which extends to the vicinity of the middle of the molten zone, becomes gradually thinner downwardly along its length. The remaining unmolten or polycrystalline portion occasionally extends into close proximity to or contact with a single-crystal portion formed below the polycrystalline portion, thereby degrading the quality of the single crystal. In addition, the remaining polycrystalline portion may influence the molten liquid flow or others in the molten zone, causing the variation of cross-sectional resistivity of the growing crystal. Particularly, this tendency outstandingly appears when the diameter of the polycrystalline rod 2a is relatively large.
The molten zone and the portion of the semiconductor rod 2 which is being melted on the polycrystalline side has a configuration which enlarges radially upward from the neck portion of the molten zone. This configuration allows heat to easily dissipate and, therefore, the outer portion where melting starts on the polycrystalline side is kept at a low temperature. As a result, an icicle-like projection 3 may be formed there. As the semiconductor rod 2 moves downwardly, the projection 3 collides with the coil 1, thereby causing electrical discharge to start there or disabling the relative movement of the coil 1 and the semiconductor rod 2.